1. Field of the Invention
The present invention relates to an image forming apparatus wherein an image on an image bearing member is transferred onto a transfer material born by a transfer material bearing member. The present invention can be applied, for example, to image forming apparatuses of electrophotographic type or electrostatic type and particularly to electrophotographic color copying machines and printers, wherein a plurality of images having different colors are formed on an image bearing member such as an electrophotographic photosensitive member and the images are successively transferred onto the same transfer material.
2. Related Background Art
In the past, there have been proposed various image forming apparatuses (so-called "color image forming apparatus") wherein toner images having different colors are formed at respective image forming portions and the toner images are successively transferred onto the same transfer material. Among them, a multi-color electrophotographic apparatus is most popular. In FIG. 10, a toner image is formed on a photosensitive member (image bearing member) 1 at each image forming portion and the toner image is transferred onto a transfer material 6 carried by a transfer belt (transfer material bearing member) 8.
With this arrangement, in order to effectively transfer the toner image formed on the image bearing member 1 such as a photosensitive drum to the transfer material 6, a regulating member for shielding or blocking the transfer electric field is often arranged upstream of the contact position between the image bearing member 1 and the transfer material bearing member 8. That is to say, in order to effect the transferring to faithfully reappear or reproduce the toner image formed on the image bearing member 1, it is necessary to regulate the transfer electric field, thereby preventing the scattering of toner (color particles forming the toner image).
To this end, a regulating member may be provided on the collotron which is usually used. However, for the most simple construction, as shown in FIG. 10, the use of a an electrode of a brush type or a plate type (particularly, plate-shaped electrode) is well known. By using such an electrode, the transfer electric field can be applied with high accuracy.
On the other hand, when the plate-shaped electrode is used, during the transferring operation, if the urging pressure between the image bearing member 1 and the transfer sheet 6 is too great, the toner image will be strongly pressed on the image bearing member 1, with the result that the toner image is not transferred to the transfer material 6 but remains on the image bearing member 1. To avoid this, when the plate-shaped electrode is used as a transfer charge means, the electrode is usually contacted with the image bearing member in a manner as shown in FIG. 10 to uniformly contact the electrode with the image bearing member with low pressure as much as possible. That is to say, the plate-shaped electrode 4 comprises a plate-shaped conductive blade (electric field applying means) 401 and an electric field regulating member 403 disposed at an upstream side of the transfer position, and a predetermined voltage is applied to the blade 401 via an electrode 402, and the blade 401 is abutted against the transfer material bearing member 8 along a shifting direction of the transfer material bearing member.
On the other hand, when a roller-shaped electrode is used as the transfer charge means, if the transfer material bearing member is urged by the electrode too strongly, a transfer void occur due to the compression of toner.
Although the urging pressure between the image bearing member 1 and the transfer material 6 can be reduced by using the above-mentioned conventional plate-shaped electrode 4, since there is the relative movement between the plate-shaped electrode 4 and the transfer material bearing member 8 contacted with the electrode, these elements are worn. Particularly, when the conductive blade 401 of the plate-shaped electrode 4 is made of resin having high coefficient (.mu.) of friction, significant wear occurs, with the result that the abutment area between the blade and the transfer material bearing member is increased and the abutment area is shifted toward the upstream side, thereby causing the discharging phenomenon. Since the blade is made of conductive material, if the blade is worn, the conductive powder will be adhered to other electrodes, substrates, or wirings, thereby causing the discharging phenomenon, current leak or the like.
On the other hand, if the blade is made of a hard material such as metal to reduce the wear of the blade, the transfer material bearing member or the image bearing member will be damaged when the transfer charge means is contacted with the transfer material bearing member. This may happen when in the inoperative condition such as the case where the transfer material jammed due to the poor feeding is removed, the apparatus is assembled or the interior of the apparatus is cleaned, and also, for example, may happen in the case where the apparatus is designed so that the transfer charge means can be separated from the transfer material bearing member in order to release the urging force between the image bearing member and the transfer material bearing member, for preventing the transferring of the residual charges to the image bearing member, for preventing the undesirable friction between the image bearing member and the transfer material bearing member and/or for controlling the detection of the density of toner on the image bearing member.
Further, in the arrangement as shown in FIG. 10, since the urging force depends upon the elastic force of the conductive blade 401, the fatigue and creep of the blade 401 and the cure of the rubber and resin make it difficult to maintain the stable urging force. In addition, since the transfer charge means 4 is directly contacted with the transfer material bearing member to apply the transfer electric field, the dust such as the scattered toner and paper powder is accumulated to cause the transfer unevenness, thereby making the transfer efficiency uneven.